Homemade charge controller

I have built the following circuit which uses the comparators inside a 555 timer to control the charge of a 12 V battery from a solar panel. The circuit is adjusted (via pots R1 and R2) so that when the battery voltage gets under 12 V, the relay connects the solar panel to the battery so it charges while when the battery voltage gets over 14 V, the relay connects the solar panel to a dummy resistor so it dumps energy.

When I try the circuit with the panel outputing 15 V and the battery at 10 V, the relay swithes very fast between charge and dump. Does anyone know why?